There have been devised and demonstrated various types of rangefinders for cameras such as a superposed field system, a split-field system and a system particularly adapted for single-lens reflex cameras in which part of the light rays emerging from a single lens system are directed to pass through a lens array or a pin hole so that the image of an object is split so as to detect whether the object is sharply focused or not. Ultrasonic "radar" has also been proposed and used in practice. In either of these rangefinder systems, the distance to an object is converted into an electrical signal.
These rangefinders are incorporated not only in fixed-lens cameras; that is, cameras whose lens cannot be replaced, but also for cameras such as single-lens reflex cameras capable of selectively using one of various types of photographic lenses.
In the case of the fixed-lens camera, there has been devised and demonstrated an automatic focusing system in which a photographic lens is automatically shifted with a predetermined speed in a predetermined direction and then stopped automatically in response to the occurrence of an electrical output signal from the rangefinder.
However, in the case of the interchangeable-lens camera, even when the distance-to-object remains unchanged, the distance between the interchangeable lens and the film plane is different depending upon the type of the interchangeable lens used. Therefore, when the automatic focusing system for the fixed-lens camera of the type described above is used, the lens shifting velocity must be varied depending upon the type of an interchangeable lens used. As a result, the automatic focusing system for the interchangeable-lens camera becomes extremely complex in construction. In order to overcome this problem, there has been devised and demonstrated a system in which the focusing is made manually by rotating the focusing ring and when the object is sharply focused, an electrical signal is generated to turn on a light-emitting element to show a camera operator that the object is sharply focused. In another system as disclosed in Japanese Patent Application Laid-Open No. 116030/1975, when an object is sharply focused with a rangefinder, an electrical signal is generated which represents the distance-to-object. In response to this signal, a lens stopper is displaced to a predetermined position at which the lens is stopped when the lens is shifted.
In the former system, a suitable display means or the like is turned on so that a camera operator can stop the lens at a position at which the object is sharply focused on the film plane. But due to the characteristics of the display means or the like, confirmation errors occur, so that the lens is stopped at a wrong position at which the object is not sharply focused. Setting the lens at a wrong position is also caused by an unexpected shift of the lens.
In the latter system, the measurement of the distance-to-object is carried out independently. That is, there does exist a time interval from the time when the signal representative of the distance-to-object is generated to the time when the lens stopper is displaced and stopped at a predetermined position in response to the distance-to-object signal. Therefore, for instance, when the object moves from the initial position at which the distance-to-object signal is generated to another position before the lens stopper is set to a predetermined position, the lens is set at a wrong or out-of-focus position. As a result, there arises the problem of failure in accurately tracking the object.